It didn't take me too long to clear the first three levels (lite/free/demo version of Hex Defense) in "Easy" mode. But I've been trying for days to play in "Hard" mode and can not get past even the first level, "Pinball." The highest I can make is 17 waves. Is there something wrong with my strategy? My basic approach is to make alleys to try to maximize the Laser towers. I've tried both upgrading fewer towers, as well as just planting lots of weak towers. One thing I don't understand is the blue pulser.
screenshot at http://www.flickr.com/photos/markhud/5979386743/
P.S. Update: after learning and applying the tips below, I decided the game was fun enough to buy the full version.